oidmap: Enhance the khash wrapper
pqueue: Introduce git_pqueue_peek_ahead()
tests: Add "describe" test repository

Built with the following script:

	#!/bin/sh

	test_tick () {
		sleep 10
	}

	test_tick &&
	echo one >file && git add file && git commit -m initial &&
	one=$(git rev-parse HEAD) &&

	git describe --always HEAD &&

	test_tick &&
	echo two >file && git add file && git commit -m second &&
	two=$(git rev-parse HEAD) &&

	test_tick &&
	echo three >file && git add file && git commit -m third &&

	test_tick &&
	echo A >file && git add file && git commit -m A &&
	test_tick &&
	git tag -a -m A A &&

	test_tick &&
	echo c >file && git add file && git commit -m c &&
	test_tick &&
	git tag c &&

	git reset --hard $two &&
	test_tick &&
	echo B >side && git add side && git commit -m B &&
	test_tick &&
	git tag -a -m B B &&

	test_tick &&
	git merge -m Merged c &&
	merged=$(git rev-parse HEAD) &&

	git reset --hard $two &&
	test_tick &&
	echo D >another && git add another && git commit -m D &&
	test_tick &&
	git tag -a -m D D &&
	test_tick &&
	git tag -a -m R R &&

	test_tick &&
	echo DD >another && git commit -a -m another &&

	test_tick &&
	git tag e &&

	test_tick &&
	echo DDD >another && git commit -a -m "yet another" &&

	test_tick &&
	git merge -m Merged $merged &&

	test_tick &&
	echo X >file && echo X >side && git add file side &&
	git commit -m x
object: introduce git_object_describe()